A common belief is that drug seeking is regulated by dopamine, a chemical nerve signal associated with motivating and rewarding behavior. Dopamine may still have a key role, the researchers noted, which is why they were surprised to find harmful effects of stress converging in a brain region- the dorsal raphe nucleus --where nerve cells that use serotonin are abundant. These nerve cells also project to other structures found on either side of the brain -- the nucleus accumbens -- which are thought to play roles in feeding and drug addiction. Serotonin is a chemical nerve signal that has been associated with wake and sleep cycles, mood, anger, status and aggression. 

 In explaining their study, the researchers said that the dynorphin/kappa opioid system, found in certain brain cells, can be activated either through repeated stress or by giving a chemical that triggers a receptor on the cells. Activation of this system produces what is called conditioned place aversion in mice. They avoid smells, locations or tactile sensations similar to those present during a troubling experience. Research suggests that this response is mediated by the stress-evoked release of dynorphins, the "feel bad" brain signals.

The research was supported by U.S. Public Health Services grants from the National Institute on Drug Abuse of the National Institutes of Health, and the Hope for Depression Foundation. 

Source: http://www.sciencedaily.com/releases/2009/10/091030100018.htm